An X-ray measurement apparatus of this kind is known from U.S. Pat. No. 4,442,496.
The cited Patent Specification describes an X-ray measurement apparatus whereby radiation properties of a radiation source, notably an X-ray tube, can be measured. X-rays are generated in an X-ray tube by exposing an anode to an electron beam. The maximum energy of the X-rays is determined by the maximum voltage whereby the electrons are accelerated in the X-ray tube, so that the acceleration voltage of the X-ray tube can be determined from the radiation spectrum of the X-rays. When the X-ray source is used for the irradiation of objects, the radiation dose received by the object is determined not only by the duration of the exposure, but also by the radiation energy. The radiation energy is dependent on the peak value of the acceleration voltage which is often pulse-shaped. When patients are irradiated for the formation of radiographic or fluoroscopic images, exact correspondence is required between the adjusted acceleration voltage and the voltage actually present in the X-ray tube in order to achieve high-quality and safe imaging, considering the detrimental effects of high radiation doses on the human body and the influencing of the imaging contrast by the energy of the X-rays. To this end, calibration of the peak value of the adjusted acceleration voltage, referred to hereinafter as kVp, will be necessary at regular intervals. Because the acceleration voltages amount to some tens of kV in an X-ray tube, direct measurement of the acceleration voltage is cumbersome. When use is made of a relation between the energy of the X-rays and the kVp value, a kVp value can be indirectly derived by measurement of the spectrum of the X-rays. When a filter is irradiated, a radiation beam is attenuated inter alia by the photoelectric effect and compton scattering, these interactions of X-rays and matter are dependent on the energy of the X-rays. When a beam of X-rays having an intensity I.sub.o is incident on an object having a thickness d in the direction of irradiation, a beam having an intensity I, given by I=I.sub.o exp (-.mu.(E).times.d), emanates from the object. Therein, .mu.(E) is the energy-dependent linear attenuation coefficient. For a large variety of materials the variation of .mu.(E) as a function of the energy is accurately known on the basis of measurements. When a radiation beam having an intensity I.sub.o is incident on two filters having a different thickness, d.sub.1 and d.sub.2, respectively, the logarithm of the quotient of the radiation intensities detected beyond the filters produces a value for .mu.(E).times.(d.sub.2 -d.sub.1). For the linear attenuation coefficient found an associated energy value of the X-ray beam can be looked up in a table or graph.
In addition to the measurement of the radiation energy, in the case of pulsed operation of the X-ray source the pulse shape can also be determined from the detector signals. This operation provides inter alia information concerning the operation of the acceleration voltage generator. Another important quantity to be determined is the overall dose delivered by the source. This dose is measured in rontgen, one rontgen corresponding to the amount of radiation that produces 2.08.times.10.sup.9 ion pairs per cm.sup.3 of air at standard pressure and temperature. This corresponds to an energy deposition of 0.85.times.10.sup.-2 jKg.sup.-1. For the measurement of the kVp values of the X-ray source it suffices when a maximum value can be detected for the signals produced by the detectors, so that the measurement of the kVp can be executed by means of detectors formed by photodiodes which are optically coupled to a scintillation crystal. The conversion of low light levels into an electric signal by the photodiodes and the low currents involved, being in the order of magnitude of 100 pA, do not impede these measurements. However, for dose measurements, a stronger signal amplification or a more sensitive detector will be required in order to enable also the measurement of low doses, notably when the X-ray source is used in the fluoroscopy mode. Known X-ray measurement apparatus, for example as described in a leaflet published by Victoreen Inc., Cleveland, Ohio, in 1985, concerning the "Nero 6000 B", use an air-filled ionization chamber as a detector for dose measurements, a measurement current thereof being a measure of the dose rate which is per definition measured in Rs.sup.-1. Because the amount of ionized air, and hence the sensitivity of the detector, depends on the volume of the ionization chamber, these detectors have comparatively large dimensions (some tens of cm.sup.3). The possibility of using such X-ray measurement apparatus in an automatic control mode in which dose and kVp are measured during fluoroscopy of an object, their measurement values being used in a control circuit for readjustment of the X-ray source, is not optimum, because of the comparatively large dimensions of the X-ray detector, without appreciable distortion of the X-ray image.